1. Field of the Invention
The present invention relates to a heterojunction bipolar transistor and, particularly, to an epitaxial thin-film structure of a heterojunction bipolar transistor capable of improving ruggedness of the transistor in the event of excessive input and load change.
2. Description of Related Art
A power amplifier for mobile phones is required to have high ruggedness such that it does not fail when an output load impedance of the power amplifier changes upon excessive input. For example, it is required to have high ruggedness such that it does not fail even if an output is in the mismatch state (VSWR=15:1) in the event of excessive input at +8 to +13 dB. Further, in order to reduce the cost of mobile phones, a structure that eliminates an isolator, which has been placed between an antenna and a power amplifier, is used in increasing cases. Accordingly, the ruggedness becomes increasingly important.
A heterojunction bipolar transistor (HBT) is a bipolar transistor with a heterojunction structure in which an emitter bandgap is larger than a base bandgap in an emitter-base junction. A HBT is mainly used in a power amplifier for mobile devices (particularly, mobile phones). In order to improve the ruggedness of a HBT, techniques of increasing an on-state breakdown voltage, which is a breakdown voltage during operation, are disclosed as follows. For example, Masaya Iwamoto “IEEE Transactions on Microwave Theory and Techniques”, Vol. 48, No. 12, December 2000, pp. 2383, Table 3 (which is referred to hereinafter as Iwamoto) discloses a structure that improves a collector-emitter breakdown voltage BVceo by increasing the thickness of a collector layer. FIG. 19 schematically shows an exemplary structure of a heterojunction bipolar transistor 71 using the technique taught by Iwamoto (a related art 1).
Japanese Unexamined Patent Application Publication No. 2006-060221 (Pan) discloses a collector structure that improves an on-state breakdown voltage by increasing the doping concentration of a collector layer gradually from a base layer side to a subcollector layer side. For example, the concentration of a collector layer composed of three layers increases gradually from a base layer side to a subcollector layer side. In this structure, if an electric field is applied to a collector, a depletion layer in the collector expands according to the electric field. Accordingly, a concentration of the electric field in the collector, and particularly on the subcollector side, is reduced to thereby suppress the occurrence of avalanche breakdown. With the impurity concentration and the collector layer thickness that are described in an embodiment taught by Pan, an on-state breakdown voltage (a collector-emitter breakdown voltage during operation) with a collector current density Jc of 0 to 20 kA/cm2, which is an operating current range of a mobile phone power amplifier, is reduced (cf. “collector 1” in FIG. 5 of Pan). FIG. 20 schematically shows an exemplary structure of a heterojunction bipolar transistor 72 that partly adjusts the collector layer thickness and concentration to conform with a power amplifier for mobile phones while retaining the principle of operation that is disclosed in Pan (a related art 2).
Japanese Unexamined Patent Application Publication No. 2006-203036 (Tanpo) discloses a technique for providing a semiconductor device that enables to stabilize HBT characteristics while achieving finer design rules and significant reduction in number of manufacturing process by using simultaneously-formed electrodes of emitter, base and collector electrodes. The heterojunction bipolar transistor taught by Tanpo has a collector structure that includes a lamination of an n− type GaAs layer, an n+ type GaAs layer and an n− type GaAs layer. In this structure, the doping concentration of the collector layer gradually increases or decreases toward an upper and a lower semiconductor. The concentration gradient at the interface of each layer is thus reduced, thereby stabilizing the drifting of electrons.
However, the above related arts have the following problems. Specifically, the mere increase in collector layer thickness as seen in the structure taught by Iwamoto causes the Kirk effect to become significant. As a result, an electric field in the vicinity of the base layer of the collector layer is lowered particularly. This hinders the electrons that are injected from the base layer to the collector layer from flowing with sufficient acceleration, leading to a decrease in collector current. Therefore, a knee voltage increases, which results in the degradation of the efficiency of a power amplifier.
Further, in the technique of Pan, if an electric field is applied to a collector layer, a depletion layer within the collector layer expands according to the electric field to thereby prevent breakdown. Accordingly, the width of the depletion layer constantly changes largely according to a voltage applied to the collector layer, so that base-collector capacitance also changes significantly. This causes the degradation of AM/PM distortion (phase distortion), which degrades the distortion characteristics of a power amplifier.